Speed responsive motor starting system

ABSTRACT

An electronic switching circuit for controlling the start operation of a single phase induction motor in accordance with the speed of the motor. A bilateral solid state switching device is connected in series with the motor starting reactance for activating and disabling same. A current-sensing element is coupled in circuit with the motor run winding for controlling the operation of a reed relay. When closed, the reed relay passes gating current to the solid state switching device for enabling activation of the motor starting reactance.

United States Patent [191 Fink, Jr. et al.

[ SPEED RESPONSIVE MOTOR STARTING SYSTEM [75] Inventors: Leon Fink, Jr.,Arlington; David C. Fricker, Hurst; Larry D. Thompson, Arlington, all ofTex.

[73] Assignee: ECC Corporation, Euless, Tex. [22] Filed: Mar. 12, 1971[2]] Appl. No.: 123,737

[52] US. Cl. 318/221 E, 318/221 G, 318/227 [51] Int. Cl. H021) 1/44 [58]Field of Search 318/220 R, 221 R,

[5 6] References Cited UNITED STATES PATENTS 4/1968 Lewus 318/221 E6/1968 Buiting et al 318/221 E 12/1968 Prouty 3l8/22l E 1/1970 Lewus318/221 E Primary Examiner-Gene Z. Rubinson Attorney-Giles C. Clegg,Jr., Jack A. Kanz and Richard E. Bee

57 ABSTRACT An electronic switching circuit for controlling the startoperation of a single phase induction motor in accordance with the speedof the motor. A bilateral solid state switching device is connected inseries with the motor starting reactance for activating and disablingsame. A current-sensing element is coupled in circuit with the motor runwinding for controlling the operation of a reed relay. When closed, thereed relay passes gating current to the solid state switching device forenabling activation of the motor starting reactance.

15 Claims, 4 Drawing Figures START MECHANISM PATENTEIJBU 16 I973 3,766L457 28 START MECHAN l S M START MECHANlSM sTART MECHANISM FIG. 3

I sTART I T 28 MECHANISM flz INVENTORS E LEON FINK, JR. DAVID c. FRICKERI LARRY D. OMPSON 1 SPEED RESPONSIVE MOTOR STARTING SYSTEM BACKGROUND OFTHE INVENTION This invention relates to motor starting circuits and,more particularly, to motor starting circuits especially adapted for usein the starting of single phase induction motors.

Single phase induction motors and the operation thereof are well knownin the art. Such motors are generally classified according to the typeof starting mechanism used. For example, typicalv motors are referred toas split phase, capacitor start/capacitor run, capacitor start/inductorrun, and the like. Typically, appropriate alternatingcurrent linevoltage is applied across the run winding of a single phase inductionmotor and a branch circuit containing a starting reactance is utilizedto produce starting torque. After the motor has been started and asuitable operating speed reached, the branch circuit containing thestarting reactance is disconnected from the line voltage. Thereafter,the motor continues to run with the necessary torque being provided bythe run winding.

In the past, the starting reactance has usually been connected anddisconnected by means of either a mechanical switch operated bycentrifugal force or a mechanical switch operated by a relay typesolenoid. Each of these disconnect methods is characterized by inherentdifficulties relating to size, reliability and operational tolerance.

More recently, solid state switching devices have been proposed for useas motor starting switches. In certain ones of the proposed circuits,operation of the solid state switching device is controlled as afunction of time. In others, different motor parameters, such as theamount of'current flowing through the run winding, is used to controlthe solid'state switching device so as to disconnect the startingreactance after the motor has attained a desired speed. Generally, it ispreferred that the starting reactance be disconnected after the motorhas attained a desired speed, rather than depending upon a predeterminedtime delay. Unfortunately, however, the sensing of the desired motorspeed without adversely affecting the motor characteristics has proveddifficult.

In a typical speed responsive solid state system of the type heretoforeproposed, a bilateral solid state switching device is connected inseries with the starting reactance and a current-sensing resistor isconnected in series with the run winding of the motor, thecurrentsensing resistor being connected to the gate electrode of thesolid state switching device for controlling same for enabling linecurrent to flow through the starting reactance when the motor speed isbelow the desired sult, the switching device will not immediatelycommence conduction at the beginning of each half cycle of the linevoltage. Instead, there will be a time lag caused by the lag intriggering of theswitching device. Consequently, the current flowthrough the starting reactance will have a waveform'characterized by azero level notch immediately following the beginning of each half cycle.The presence of such notches results in a decrease in the effectivepower supplied to the starting reactance, thus reducing the startingtorque applied to the motor.

Another disadvantage of solid state starting circuits of the foregoingtype is that a small significant voltage must be produced across thecurrent-sensing resistor in order to produce sufficient gate drive tocause the solid state switching device to switch to its low impedancestate. To accomplish this, the current-sensing resistor must possess acertain minimum amount of resistance. The power dissipated in thisresistance is, however, wasted and produces no useful result after themotor has started and the starting reactance has been disconnected.

It is an object of the invention, therefore, to provide a new andimproved speed responsive motor starting system which employs anelectrically-controllable solid state switching device and which enablesa fuller real-' ization of the maximum starting torque capability of themotor.

It is another object of the invention to provide a new and improvedspeed responsive motor starting system which employs anelectrically-controllable solid state switching device and whichminimizes the resistance introduced in circuit with the motor runwinding. 7 v

For a better understanding of the present invention, together with otherand further objects and features thereof, reference is had to thefollowing description taken in connection with the accompanying drawing,the scope of the invention being pointed out in the appended claims.

BRIEF DESCRIPTION OF THE DRAWING I Referring to the drawing: FIG. 1 is aschematic circuit diagram illustrating one embodiment of the presentinvention;

FIG. 2 is a schematic circuit diagram illustrating a second embodimentof the invention;

FIG. 3 is a schematiccircuit diagram illustrating a third embodiment ofthe invention; and

FIG. 4 is a schematic circuit diagram illustrating a fourth embodimentof the invention.

DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS I Referringto FIG. 1 there isshown a single phase alternating-current induction motor which includesa rotor 10, a run winding 11 and a starting mechanism 12. Run winding 11and start mechanism 12 are located in parallel circuit branches whichare connected between a pair of supply circuit means adapted to beindividually connected to different sides of an electrical power source.A first of these supply circuit means includes a power'supply conductor13 having a power supply terminal 14 at one end thereof. The second ofthese supply circuit means includes a second power supply conductor 15having a power supply terminal 16 at one end thereof. In use, terminals14 and 16 are connected to different sides of an electrical power sourcesuch as, for example, a two-wire alternating-current power line. Startmechanism 12 includes the appropriate starting reactance for initiallystarting rotation of the rotor 10. Such starting reactance may be, forexample, of the split phase type, the capacitor start/capaci- -tor runtype or the capacitor start/inductor run type.

device 20 having a pair of power electrodes 21 and 22 and a control orgate electrode 23. For sake of convenience, the power electrode 21located on the same sideof the device 20 as the gate electrode 23 willbe referred to as the cathode, while the other power electrode 22 willbe referred to as the anode. The switching device 20 is connected inseries with the start mechanism 12 by meansof its power electrodes 21and 22,

the anode 22 being connected to the start mechanism "12 and the cathode21 being connected to the lower power supply conductor 15.

When the gating current supplied to the gate elec-' trode 23 exceeds apredetermined trigger level set .by the internal characteristics of thedevice 20, the device 20 is conditioned to conduct current from onepower electrode to the other whenever the voltage across the powerelectrodes-exceeds a certain minimum value.

When in such conductive condition, the impedance between powerelectrodes 21 and 22 is of a very low value. When, on the other hand,the gating current supplied to the gate electrode 23 is less than thepredetermined trigger level, the device 20 remains nonconductive and theimpedance between the power electrodes 2lland 22 is relatively high. Thedevice 20 will switch to the conductive mode for either positive ornegative polarity gating current at the gate electrode 23, provided theamplitude of such current is above the trigger level.

The motor starting system of FIG. 1 further includes relay means,represented by a reed relay 24, having contact means, represented byreed elements 25 and 26, and a control winding 27 for controlling theopening and closing of the reed elements 25 and 26. Reed elements 25 and26 are located within an evacuated glass vial (not shown) and thecontrol winding 27 is wound around the outside thereof in the usualmanner. The relay means represented by reed relay 24must be a highlysensitive device requiring only a small amount of current to'operatesame. Such relaymeans must also possess a'very high degree ofreliability such that it can undergo tens of thousandsof closureswithout becom,-

is the only device possessing these necessary characteristics It may be,of course, that at some future date an even better form of relay devicemay be invented, in which case, such device can be used in place of thereedrelay24. v I

The contact'elernen ts 25 and 26 of the reed relay 2.4 are connected incircuit with the gate electrode'23 of the switching device 20 forsupplying gating current thereto when the contact elements '25 and 26are closed. More particularly, the lower contact element 26is connecteddirectly to the gate electrode 23, while the upper contact element 25 isconnected to the upper resistor 28. 1 I

The motor starting system also includes means responsive to currentflowing through the motor run winding 11 for supplying control currentto the relay controlwinding 27. This circuit means includes acurrent-sensing resistor30 connected in series with the motor runwinding 1 l. Resistor 30 has a very low value of resistance on the orderof a few tenths of an ohm. In the FIG. 1 embodiment, the means forsupplying conpower supply conductorl3 by way of a current-limiting trolcurrent to the relay control winding 27 further includes rectifiercircuit means 31 for supplying unidirectional control current to therelay winding 27. This rectifier circuit means 31 includes a diode 32and a capacitor 33 connected in series across the current-sensingresistor 30. The capacitor 33 is, in turn, connected across the relaycontrol winding 27.

Considering now the operation of the FIG. 1 embodiment, the magnitude ofthe current flowing through the 'motor run winding 1 1 varies inverselywith the speed of rotation of the rotor 10. When rotor. is stationary,

" the current flow is faily heavy. As the rotor 10 picks up speed, thecurrent flow decreases. Current-sensing resistor 30 develops thereacrossa voltage drop which is proportional to the magnitude of the run windingcurrent. This alternating-current voltage drop across resistor 30 isrectified by the diode 32 and smoothed by the capacitor 33 to developacross the capacitor33 a unidirectional or direct-current voltage havinga magnitude which is proportional to the run winding current. When thevoltage across capacitor 33 reaches a certain level, the-energizingcurrent flowing through the relay COll trol winding 27 becomessufficient to cause the reed elements 25 and 26 to moveinto contactwithone another. The reed .elements 25 and 26 'remain'closed in this mannerso long as the voltage across capacitor 33 exceeds the relay openinglevel. With reed relays, this opening level'voltage is typically a voltor two less than f the voltage level initially required to close therelay contacts 25 and 26. These'openingand closinglevels are determinedby the characteristics of the reed relay cathode 21 and to the lowerpower supply conductor 15 during one half cycle of thealternating-current line voltage. During the next half cycle, thedirection of the current flow in this gate electrode circuit'isreversed. As mentioned earlier, either polarity or direction of currentflow in the gate electrode circuit is sufficient to trigger theswitching device 20 so long as the current amplitude is above thetrigger level. The value of resistor 28 is selected so that the gatingcurrent exceeds this trigger level for practically the entire timeduring'each halfcycle. Y 1 v There is, of course, a short interval. atthe beginning ofeach half cycle during which the/gating current is belowthe. trigger level. The circuit is contructed so as to keep this timeinterval from delaying the triggering of the device 20; This isaccomplished by making the amplitude and the phase angle of the gatingcurrent relative to the line voltage applied 'acrossthepower electrodes21 and 22 such that the gating current amplitude is greater than thetrigger level at or very immediately after thefline voltage crosses. theiero' axis. In other words, the gate current amplitude is causedtoexceed the trigger level at or before the instantthe power electrodevoltage exceeds the value required to cause current flow between thepower electrodes 21 and 22. Thus, when the reed elements 25 and 26 areclosed together, the switching device 20 is conductive practically theentire time and current flows through the start mechanism 12 in verynearly the same manner as if the lower sideof the start mechanism wereconnected directly to the lower power supply conductor 15.

The flow of current through the start mechanism 12 provides the initialstarting torque for initiating rotation of the rotor 10. As the rotorspeed continues to increase, a point is soon reached when the voltagedrop across the current-sensing resistor 30 is insufficient to keep thereed elements 25 and 26 closed. Consequently, such elements open. Thisdiscontinues the flow of gating current tothe gate electrode 23 which,in turn, renders the switching device 30 nonconductive for the remainderof the time. This turns off and disables the current flow through thestart mechanism 12. After this point is reached, the torque for drivingthe rotor is provided by the run winding 11.

The FIG. 1 embodiment offers several advantages over the heretoforeproposed solid state system described above. For one thing, theso-called notch effect is considerably reduced. This occurs because thegating current for the switching device 20 is derived directly "from thepower supply conductors 13 and 15 by means relay contacts 25 and 26.Thus, current-sensing resistor 30 can have a lower value of resistance.This decreases the power dissipated in the resistor 30, which powerdissipation serves no useful purpose and is wasted after the motor hasstarted and the starting mechanism 12 has been disconnected.

A further advantage results from the hysteresis effect represented bythe difference in opening-and closing voltage levels for the reed relay.24. More particularly, since more voltage is required to initially closethe reed elements 25 and 26 than to keep them closed, there is lesslikelihood of the start mechanism 12 being turned back on accidentlyonce the rotor 10 has reached the desired minimum speed. I

As indicated in the FIG. 2 embodiment, the currentlimiting resistor 28may be connected to the anode electrode 22 of the switching device 20instead of to the upper power supply conductor 13. An advantage of thisFIG. 2 connection is that when the switching device 20 is switched toits low impedance condition, the voltage appearing across the gateelectrode circuit formed by resistor 28 and relay contacts 25 and 26will be relatively small/This reduces the power dissipated in theresistor 28. In some instances, it is also practical in the FIG. 2embodiment to omit the resistor 28 and instead connect the reed element25 directly to the anode 22. This is because the switchingcharacteristics of the switching device 20 are normally such that thedevice is switched from the high impedance state to the low impedancestate in such an extremely short period of time that current surgesthrough the reed elements 25 and 26 are practically non-existent.

As indicated in the embodiment of FIG. 3, it is also practical in somecases to omit the rectifier circuit formed by diode 32 and capacitor 33and to instead connect the relay control winding 27 directly across thecurrent-sensing resistor 30. This will depend upon the characteristicsof the particular type reed relay which is used. If such reed relay canbe operated with alternating current without causing reed chatter, thenthe diode 32 and capacitor 33 can be eliminated.

Referring to FIG. 4, there is shown the case where the current-sensingelement coupled in circuit with the motor run winding 11 takes the formof a transformer mechanism. In the illustrated embodiment, thistransformer mechanism comprises a current transformer 40 having a firstwinding 41 connected in series with the motor run winding 11 and asecond or output winding 42 inductively coupled to the first winding 41.One side of the output winding 42 is connected to the rectifier diode32, while the other side of the output winding 42 is connected to thelower power supply conductor 15. Current transformer 40 is constructedto provide a very, very low impedance in series with the motor runwinding 11. Thus, the use of current transformer 40 serves to provideeven less resistance in series with the motor run winding 11 than doesthe resistor 30 of the earlier embodiments. Thus, the undesired powerdissipation is further reduced. On the other hand, a current transformerwill normally cost somewhat more than a resistor. Consequently, thecurrent transformer embodiment can be used to best advantage with higherhorsepower motors where the saving in power loss will tend to offset theadded cost of the current transformer. f 4

Instead of using a current transformer in the manner shown in FIG. 4, itwill sometimes be more feasible to use a modified form of currenttransformer wherein the primary winding 41 is omitted and the outputwinding 42 is instead placed in close physical proximity to the motorrun winding 11 so as to be inductively coupled thereto. Thus, in termsof FIG. 4, winding 41 may be omitted and winding 42 wound alongside ofsome of the turns of run winding 11 and located therewith inside themotor housing.

While there have been described what are at present considered to bepreferred embodiments of this invention, it will be obvious to thoseskilled in the art that various changes and modifications may be madetherein without departing from the invention, and it is, therefore,intended to cover all such changes and modifications as fall within thetrue spirit and scope of the invention.

What is claimed is:

1. A speed responsive motor starting system comprising:

a. a motor including a run winding and a start mechanism; b. a bilateralswitching device having a pair of power electrodes and a controlelectrode;

c. relay means having contact means and a control winding forcontrolling the opening and closing of the contact means;

d. means connecting the switching device by its power electrodes inseries' with the motor start mechanism; v

e. means connecting the relay contact means in circuit with the controlelectrode of the switching device for supplying gating current theretowhen the relay contact means areclosed; and,

includes a'pair of supply circuit means adapted to be 7 individuallyconnected to different sides of an electrical power source and whereinone side of the relay contact means is coupled to one of the supplycircuit means and the other side of the relay contact means is coupledto the control electrode of the switching device.

4."A'system as defined in claim 3 wherein the relay contact circuitrunning from the supply circuit means to the control electrode includesa resistor in series therein.

'5. A'system asdefined in claim 1 wherein one side of the relay contactmeans is coupled to one of the power electrodes of the switching deviceand the other side of the relay contact means is coupled to the controlelectrode of the switching device.

6. A system as defined in claim 5 wherein the relay contact circuitrunning from the power electrode to the control electrode includes aresistor in series therein.

7. A system as defined in claim 1 wherein the means for supplyingcontrol current to the relay control winding includes a current-sensingresistor connected in series with the motor run winding and circuitmeans connected between the current-sensing resistor and the relay"control winding for supplying control current to the relay controlwinding.

8. A system as defined in claim 7 wherein the circuit means connectedbetween the current-sensing resistor and the relay control windingcomprises rectifier circuit means for supplying unidirectional controlcurrent to the relay control winding.

9. A system as defined in claim 8 wherein the rectifier circuit meanscomprises a diode and a capacitor connected in series across thecurrent-sensing resistor and circuit means connecting the capacitoracross the relay control winding. i

, 10. A system as defined in claim 1 wherein the means for supplyingcontrol current to the relay control winding includes transformer meansresponsive to current flowing through the motor run winding and havingan output winding for providing a signal indicative of such current flowand circuit means connected between the transformer output winding andthe relay control winding for supplying control currentto the relaycontrol winding. I

1 1. A system as defined in claim 10 wherein the circuit meansconnectedbetweenth'e transformer output winding and the relay controlwinding comprises rectifier circuit means for supplying unidirectionalcontrol current to the relay control winding.

. 12. A system as defined in claim 11 wherein the rectifier circuitmeans includes a diode and a capacitor connected in series across thetransformer output winding and circuit means connecting the capacitoracross the relay control winding.

13. A system as defined in claim 1 wherein the means for supplyingcontrol current to the relay control winding includes a currenttransformer having a first winding connected in series with the motorrun winding and a second winding inductively coupled to the first winding and circuit means connected between the second transformer windingand the relay control winding for supplying control current to the relaycontrol winding.

14. A system asdefined in claim 13 wherein the cirrelay control winding.

1. A speed responsive motor starting system comprising: a. a motorincluding a run winding and a start mechanism; b. a bilateral switchingdevice having a pair of power electrodes and a control electrode; c.relay means having contact means and a control winding for controllingthe opening and closing of the contact means; d. means connecting theswitching device by its power electrodes in series with the motor startmechanism; e. means connecting the relay contact means in circuit withthe control electrode of the switching device for supplying gatingcurrent thereto when the relay contact means are closed; and, f. meansresponsive to current flowing through the motor run winding forsupplying control current to the relay control winding.
 2. A system asdefined in claim 1 wherein the relay means comprises a reed relay.
 3. Asystem as defined in claim 1 wherein the system includes a pair ofsupply circuit means adapted to be individually connected to differentsides of an electrical power source and wherein one side of the relaycontact means is coupled to one of the supply circuit means and theother side of the relay contact means is coupled to the controlelectrode of the switching device.
 4. A system as defined in claim 3wherein the relay contact circuit running from the supply circuit meansto the control electrode includes a resistor in series therein.
 5. Asystem as defined in claim 1 wherein one side of the relay contact meansis coupled to one of the power electrodes of the switching device andthe other side of the relay contact means is coupled to the controlelectrode of the switching device.
 6. A system as defined in claim 5wherein the relay contact circuit running from the power electrode tothe control electrode includes a resistor in series therein.
 7. A systemas defined in claim 1 wherein the means for supplying control current tothe relay control winding includes a current-sensing resistor connectedin series with the motor run winding and circuit means connected betweenthe current-sensing resistor and the relay control winding for supplyingcontrol current to the relay control winding.
 8. A system as defined inclaim 7 wherein the circuit means connected between the current-sensingresistor and the relay control winding comprises rectifier circuit meansfor supplying unidirectional control current to the relay controlwinding.
 9. A system as defined in claim 8 wherein the rectifier circuitmeans comprises a diode and a capacitor connected in series across thecurrent-sensing resistor and circuit means connecting the capacitoracross the relay control winding.
 10. A system as defined in claim 1wherein the means for supplying control current to the relay controlwinding includes transformer means responsive to current flowing throughthe motor run winding and having an output winding for providing asignal indicative of such current flow and circuit means connectedbetween the transformer output winding and the relay control winding forsupplying control current to the relay control winding.
 11. A system asdefined in claim 10 wherein the circuit means connected between thetransformer output winding and the relay control windIng comprisesrectifier circuit means for supplying unidirectional control current tothe relay control winding.
 12. A system as defined in claim 11 whereinthe rectifier circuit means includes a diode and a capacitor connectedin series across the transformer output winding and circuit meansconnecting the capacitor across the relay control winding.
 13. A systemas defined in claim 1 wherein the means for supplying control current tothe relay control winding includes a current transformer having a firstwinding connected in series with the motor run winding and a secondwinding inductively coupled to the first winding and circuit meansconnected between the second transformer winding and the relay controlwinding for supplying control current to the relay control winding. 14.A system as defined in claim 13 wherein the circuit means connectedbetween the second transformer winding and the relay control windingcomprises rectifier circuit means for supplying unidirectional controlcurrent to the relay control winding.
 15. A system as defined in claim14 wherein the rectifier circuit means includes a diode and a capacitorconnected in series across the second transformer winding and circuitmeans connecting the capacitor across the relay control winding.